Problem: 8 pencils cost $15.04. Which equation would help determine the cost of 13 pencils?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 8 pencils. We want to know the cost of 13 pencils. We can write the numbers of pencils as a proportion: $\dfrac{8}{13}$ We know 8 pencils costs $15.04. We can let $x$ represent the unknown cost of 13 pencils. The proportion of these costs can be expressed as: $\dfrac{\$15.04}{x}$ The cost changes along with the number of pencils purchased, and so the two proportions are equivalent. $\dfrac{8}{13} = \dfrac{\$15.04}{x}$